Diffusion joining method and a paste used therefor

ABSTRACT

The present invention relates to a diffusion joining method of Cu or Cu alloy surface and a conductive paste used therefor, especially applied to a method for preparing a multilayer printed wiring board. The present invention is characterized by providing the contacting surfaces with a layer selected from the group comprising a noble metal thin layer, a metal oxide remover layer and a conductive paste layer mainly consisting of a Cu or Cu alloy particles and the metal oxide remover; and pressing the contacting metal surfaces at a temperature higher than 170° C. more or less whereat the Cu atom at interface of the Cu or Cu alloy surfaces to be joined becomes to be able to diffuse, to give a joined metal body.

This is a divisional of application Ser. No. 08/071,400, filed Jun. 4,1993, now U.S. Pat. No. 5,439,164.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a new diffusion joining method used forjoining Cu or Cu alloy contacting surfaces at a low temperature in thefield of metallurgical technology and electronic engineering, which maybe applied particularly to a method for preparing a multilayer printedwiring board and also relates to a paste used in the method.

2. Description of the Prior Art

Hitherto, in order to join at least two contacting metal surfaces therehas been used widely a joining method, such as soldering. In theelectronic engineering, there has been sometimes used an ultrasonicjoining method, but in general the reliability of electric connectioncould not be obtained by any other method than the soldering method.

However, in the soldering method a contacting metal component such aslead or tin different from copper of the surfaces to be connectedsometimes causes generation of corrosion in the joint and also causesformation of intermetallic compound, which causes the problems of afaulty joint in which cracks easily generate by repeating stress. Theaforementioned ultrasonic joining method does not produce the aboveproblems, but causes the problem of only a small connecting area whichcan not be applied to the usual metal joint.

Therefore, a first object,of the present invention is to provide a newmethod for joining the contacting of Cu or Cu alloy surfaces which issuperior in the joint reliability.

Further, many kinds of resin dispersive type conductive pastes have beenused for connection of electronic parts or circuits and formation ofelectroconductive paths. However, this causes a problem of the jointhaving a relatively high resistance and thus tends to indicate non-ohmiccharacteristic by which the resistance increases in a low current range,so that use of the conventional conductive paste is limited.

Therefore, a second object of the present invention is to provide a newconductive paste which can brings the above joining method intopractice.

On the other hand, the methods used for connecting via holes of amultilayer printed wiring board in the electronic circuits areclassified into two groups. One is a method for forming a conductivepath in the via hole by means of electroplating, the other being amethod for filling the via holes with the conductive paste which isreferred to an Ag through hole method.

However, the electroplating method can provide the via hole which issuperior in conductivity and reliability while it requires manyprocesses and facilities for electroplating or washing drainage ofby-products of the process, which disadvantageously is costly. The othermethod can provide an easy-making and inexpensive Ag through hole whileit has a relatively high resistance and thus tends to indicate non-ohmiccharacteristic by which the resistance increases in a low current range,so that use of the conventional conductive paste is limited.

Therefore, a third object of the present invention is to provide amethod for preparing a multilayer printed wiring board at a low cost andhaving a low resistance and a high reliability.

SUMMARY OF THE INVENTION

As a result of our researches, we surprisingly found that Cu atoms atthe contacting interface begin to diffuse from one contacting surfaceinto the other contacting-surface in an air atmosphere even at atemperature of about 170° C. if a metal oxide layer is not formed at thecontacting surfaces.

According to the first aspect of the present invention, therefore, onthe basis of the inventive acknowledgement, there is provided a methodfor joining at least two contacting Cu or Cu alloy surfaces at a lowertemperature by diffusing Cu atoms into an opposite metal surface fromthe Cu or Cu alloy surface, which comprises:

providing the Cu or Cu alloy surface with a layer selected from thegroup comprising a noble metal thin film, a coating of metal oxideremover and a coating of a conductive paste consisting of a Cu or Cualloy particles and the metal oxide remover; and

pressing the contacting metal surfaces at a temperature higher thanabout 170° C. whereat the Cu atoms at interface of the Cu or Cu alloysurfaces to be joined can be diffused, to give a joined metal body.

In an embodiment according to the present invention, it is preferablethat one of the contacting metal surfaces is Cu or Cu alloy and coatedwith a coating layer of the metal oxide remover, while the other surfaceis of noble metal or alloy thereof or of any other metal provided withthe thin layer of noble metal or alloy thereof by means ofelectroplating, evaporating or sputtering.

In the present invention, the metal oxide remover is preferably selectedfrom the group comprising carboxylic acids and polymers or oligomershaving carboxyl groups.

Further, according to a second aspect of the present invention, there isprovided an electroconductive paste used for joining contacting metalsurfaces at a lower temperature by diffusing Cu atoms into an oppositemetal surface from the Cu or Cu alloy surface, which comprises metalparticles selected from the group comprising noble metal, Cu and alloythereof, a metal oxide remover and if necessary a binding resin.

In an embodiment of the electroconductive paste, any surface activeagent available in the soldering technology may be used as the metaloxide remover and it is preferable that it is selected from the groupcomprising carboxylic acids and polymers or oligomers having carboxylgroups. It is more preferable that the metal oxide remover acts as thebinding resin. The Cu or Cu alloy particles are preferably of sphericalform since Cu atoms diffusion is easy to occur.

Furthermore, according to the third aspect of the present invention, thediffusion joining method is applied to a method for preparing amultilayer printed wiring board. The method comprises: a step ofproviding at least two foils of Cu or Cu alloy; a step of interposingeach insulating adhesive layer between each pair of the foils; a step ofmaking via holes at desired positions of the each insulating adhesivelayer; a step of imprinting the via holes with a conductive paste and astep of heat-pressing them to electrically connect the foils through thevia holes imprinted with the conductive paste, and is characterized inthat the conductive paste mainly consists of conductive metal particlesselected from the group comprising Ag, Cu and alloys thereof and a metaloxide remover while the heat pressing is carried out at a temperaturehigher than about 170° C. whereat the Cu atom can be diffused.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 is a schematic view showing a joining process for preparing adouble-layer printed wiring board according to the prurient invention;

FIG. 2(a) is a perspective view of the double-layer printed wiring boardprepared by the process shown in FIG. 1, and FIG. 2(b) is a perspectiveview of the board after etching;

FIG. 3(a) is a schematic view showing a joining process for preparing afour layered printed wiring board from the double layered printed boardshown in FIG. 2(b);

FIG. 3(b) is a schematic view showing a joining process for preparing asix layered printed wiring board from the double layered printed boardshown in FIG. 2(b);

FIG. 4 is a schematic view showing a imprinting process with use ofpeelable films;,

FIG. 5 a graph of Cu diffusion of silver electroplated Cu/Silverinterface (180° C., 30 min, Pressed) at XMA analysis in Example 5;

FIG. 6 is a graph of Cu diffusion of deoxidized Cu/Silver interface(180° C., 30 min, Pressed) at XMA analysis in Example 10;

FIG. 7 is a graph of Cu diffusion of deoxidized Cu/Silver interface(240° C., 30 min, Pressed) at XMA analysis in Example 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The joining method of the present invention can be applied to contactingmetal surfaces of Cu or Cu alloy such as an electric copper, a rolledcopper, bronze, brass, beryllium copper, gun metal. The metal parts tobe joined may be of any form such as a plate, a foil, a wire, a rod andso on, but it is necessary that the surfaces to be joined are subjectedto a coating treatment with the metal oxide remover or noble metalbefore joining.

In the case of noble metal plating, there are known many kinds ofpretreatment for removing oxide layer before plating, which are an acidetching method, an mechanical grinding method and heating method in areducing atmosphere, from which a suitable method may be optionallyselected depending on factors such as the shapes to be joined and therequired characteristics. For example, in the case of a continuous filmsuch as a copper foil, a wet continuous process may be applied, while inthe case of small parts such as one body elements, a reducing oven maybe used.

Following the oxide layer removing treatment, a thin noble metal layer(or plating) is deposited. The noble metal layer prevents the surfacefrom oxidizing. Otherwise, the metal oxide formed in the followingheating process may prevent Cu atoms from diffusing during the joiningprocess. Contrary to this invention, the conventional Ni plating forpreventing the surface from oxidizing, prevents Cu atoms from diffusing.Therefore, the noble plating on the clean Cu or Cu alloy surface isessential to make Cu atoms diffuse. Even a small thickness of the noblemetal plating is sufficient to prevent the contacting surface fromoxidizing and promote diffusion of Cu atoms from one contacting surfaceinto the opposite contacting surface. As described below, a noble metallayer of 100 Å thickness is sufficient for that purpose. The noble metallayer of noble metal large of a larger thickness does not prevent thecontacting surfaces from joining by the Cu atom diffusion, but itdisadvantageously is high in cost. Further, in the case of using silveras the noble metal, silver thick layer causes silver migration, whichresults in lowering of reliability of the joint.

The noble metal plating method, used is any of the known methods, suchas electroplating, chemical plating, vacuum deposition, sputtering. Asuitable method can be optionally selected according to the shape to bejoined. The noble metal includes gold, silver, platinum, palladium andrhodium. Considering the cost, gold and silver are suitable. In the caseof silver, there sometimes occurs color change of the plating surface bysulfidization, but such a change does not adversely affect the joint.

In place of the noble metal plating, a metal oxide remover may be coatedon the surface of Cu or Cu alloy when joining the surfaces. The metaloxide remover means an active agent which reacts with Cu oxide and makesthe Cu or Cu alloy's surface into a pure metal surface. Especiallycarboxylic acid and polymers or oligomers containing carboxyl groups arepreferred. Examples of carboxylic acid includes linear or side chaintype saturated fatty acid such as caproic acid, enanthic acid, caprylicacid, 2-ethylhexanoic acid, stearic acid and similar acids; unsaturatedfatty acid such as oleic acid, linoleic acids, linolenic acids and soon; mono- or multi-basic carboxylic acid or amino carboxylic acid suchas abietic acid, succinic acid, malonic acid, aspartic acid and similaracids; esters thereof. Examples of the polymers or oligomers includepolymers or copolymers of acrylic acid, copolymers of maleic acid, alkydresin, polybutadiene having terminated or pendant carboxyl groups andsimilar polymers or copolymer.

The remover is coated directly or in a solution to Cu or Cu alloysurfaces. The other examples include a conventional active agent used asflux in the soldering such as hydrochloric acid, formic acid, aceticacid, lactic acid, primary, secondary and tertiary amines and theirsalts such as dimethylamine hydrochloride. In the case of usinginorganic active agent, cleaning for removing the agent is necessaryafter the joining process.

While even thick oxide layer can be removed by solution action of theremover, it is preferred in such a case that the oxide layer has alreadybeen removed before coating in a similar way to the electroplating case.

The temperature required for joining the Cu or Cu alloy surface is morethan about 170° C. When it is lower than that temperature, Cu atoms cannot diffuse to the opposite surface, resulting in no bonding between thecontacting surfaces. On the other hand, while there is not present anupper limit of the temperature, the joining process is preferablycarried out at as a low temperature as possible since Cu or Cu alloy iseasily oxidized. Higher temperature in the practicable range causesprompt joining. If possible, an ultrasonic method may be applied at thesame time of the heat-pressing process.

The pressing for the contacting surfaces does not require an extremehigh pressure other than that needed to make the opposite surfaces joinbe contacted to each other. It is preferred that the roughness at thecontacting surface is as small as possible in order to lower thepressure needed for such a joining. Even if the contacting surface has abad roughness, Cu or Cu alloy can be deformed to be able to be joinedbecause of its low rigidity. Further, insertion of a ductile metal, suchas gold or silver, between the Cu or Cu alloy contacting surfaces causesthe contacting area to be increased and the joint strength to beimproved even by means of the lower pressing pressure.

The joining method according to the present invention may be applied notonly to plates or boards but also to powders or particles. Therefore,the same joining and method can be applied to a joint or connectionformed by a conductive paste. That is, the conductive paste used forjoining contacting metal surfaces at a lower temperature can be preparedby using the metal oxide remover as a binding resin of the conductivepaste or adding the metal oxide remover to the binding resin togetherwith Cu or Cu alloy particles. With use of the conductive paste, theconductive paths can be formed by coating the paste on a substrate or byfilling the via holes and heating it under the pressure and indicatesohmic characteristics whereby the resistance never increases even in alower current range.

The conductive paste according to the present invention is characterizedin that it mainly consists of Cu or Cu alloy particles is theelectroconductive component and the metal oxide remover for removing thesurface oxide layer. As the other component, if necessary, many kinds ofbinding resins and additives may be added in order to improve printingproperties of the paste.

The particles may be preferably of spherical form in order to facilitatethe application of local pressure on the contacting surface of theparticles, but it is not necessary to have a true sphere because theshape acts only to realize the local contacting state.

Examples of the metal oxide remover acting as the binding resin includepolymers or oligomers having carboxyl groups.

Examples of the binding resin also include a thermoplastic resin andthermosetting resin used in the conventional conductive paste such asacrylic resin, epoxy resin, phenolic resin and so on.

The polymer or oligomer having carboxyl groups may be used together withthe binding resin, in which if it is epoxy resin, the polymer oroligomer acts as a hardening agent because it has carboxyl groups.

The conductive paste may be used as an assisting agent for joining Cu orCu alloy surface as well as use a joint for the electric purpose.Especially, if the contacting surface has a bad roughness, the pastealso acts to make the surface flat.

The joining method of copper and copper alloy according to the presentinvention can be used for the manufacture of a multilayer printed wiringboard. The following description will be directed to the manufacture ofthe multilayer printed wiring board with reference to the accompanyingdrawings.

Referring to FIG. 1 (a), the electric conductive paste 3 mentioned aboveis imprinted on a hole formed at a given position of the electricinsulating adhesive 1 by using, for example, a squeegee 4. The electricinsulating adhesive 1 is interposed between two copper foils 5 and 5'and then is pressed at a temperature higher than 170° C. as shown inFIG. 1 (c). In such a way, it is possible to form a substrate which hascopper foils applied to both sides thereof and which has many via holesformed at necessary positions as shown in FIG. 2 (a). The substratewhich has copper foils applied to both sides thereof is etched atunnecessary parts with a usual etching method and is formed into asubstrate having circuits formed at the both sides thereof as shown inFIG. 2 (b). The electric conductive particles in the via hole or holes 6are joined by a metallic bond as shown above and have a low electricalresistance. Further, the electric conductive particles in the via holeare joined to the copper films through a metallic bond. Therefore, it ispossible to obtain a double layered printed wiring board which issuperior in the reliability. The double layered printed wiring boardaccording to the present invention can be manufactured in a simplemanner similarly to that of a silver through hole printed wiring board.In addition, the via holes are formed by using a metallic bond.Therefore, the via hole according to the present invention has a lowelectric resistance similarly to that of the via hole formed by anelectric plating method and further, has a low electric resistance at alow current range, thereby permitting a wide use for any circuit. Pluraldouble layered printed wiring boards 8, 8' and 8" are combined withplural electric insulating adhesives 9 and 9' having electric conductivepaste imprinted at the necessary parts thereto and then pressed togetherat a temperature higher than 170° C. to form a multilayer printed wiringboard having four layers or six layers. A multilayer printed wiringboard having more layers can be formed by increasing the number of thedouble layered printed wiring boards and the insulating adhesives.

The electric conductive paste used in the manufacture of the multilayerprinted wiring board according to the present invention is characterizedin that it essentially consists of electric conductive particles, suchas noble metals or copper and their alloys, and a metal oxide remover.In addition to these components, the electric conductive paste isincorporated with a binder resin and other necessary additives forimproving the printing ability. It is necessary for manufacture of themultilayer printed wiring board that the electric conductive pasteincluding a solvent should be dried sufficiently because the paste maybe sealed in the board.

In connection with the manufacture of the multilayer printed wiringboard according to the present invention, it is preferable that theelectric conductive particles are of a spherical form which permits thecontact point between conductive particles to be pressed predominantly.In view of the purpose to execute the predominant pressure at thecontact point between the electric conductive particles, the sphericalform does not necessarily designate a perfectly sphere.

As mentioned above, carboxylic acids or polymers or oligomers includingcarboxyl group can be used for the metal oxide remover for use in themanufacture of the multilayer printed wiring board according to thepresent invention. Since the electric conductive paste is closedair-tightly in the substrate during manufacturing of the multilayerprinted wiring board according to the present invention, it is necessaryto pay attention to the boiling point of the added remover. When themetal oxide remover having a low boiling point remains in the board, thevapor of the metal oxide remover destroys the conductive path in the viahole during the soldering step. Therefore, it is preferable that themetal oxide remover such as fatty acid having a low molecular weight isreacted with a resin to form a stable compound through a chemicalreaction, such as epoxy resin and so on.

There have been known various imprinting methods such as a screenprinting method or a pin method for imprinting the electric conductivepaste mentioned above into the hole formed at a given position of theinsulating adhesive. FIG. 4 shows another method to use a peelable film10. The peelable film 10 is applied to the insulating adhesive 1 andthen is drilled together with the insulating adhesive 1. The electricconductive paste 3 is imprinted on the peelable film 10, and then thepeelable film 10 is peeled off. This method prevents the unnecessaryportion of the adhesive from being contaminated with the electricconductive paste. When the electric conductive paste consists of copperor its alloy, it is possible to etch off the electric conductive pasteadhered to unnecessary portion other than the via hole.

The pressing process can be carried out at a temperature higher than170° C., preferably higher than 180° C. When the pressing process iscarried out at a temperature lower than 170° C., the lower temperaturedoes not promote the metal atom diffusion process between the metalparticles of the electric conductive paste. As a result, even if the viahole has a low electric resistance at initial period, it shows the breakdown or an increase in the electrical resistivity during the reliabilitytest.

The necessary pressing pressure varies with the kind of the metalparticles used. The lower pressure is allowable with the electricconductive particles having a higher ductility such as gold or silver.On the other hand, a higher pressure is required for the electricconductive powders hardly deformed such as copper or its alloys.

The following description will be directed to preferred embodiments.

EXAMPLE 1

A copper plate having a thickness of 0.5 mm is previously degreased andetched lightly with an aqueous solution of sulfuric acid and hydrogenperoxide. After water washing, the copper plate is electrically platedwith silver film by a conventional electric plating method. The platingsolution comprises a non-cyanide type commercially available; the usedanode is a silver plate; and the current density is 0.5 A/dm². Theresultant silver plating film has a thickness of 500 angstroms and iscolored silver white at the copper surface.

Two copper plates having silver plating film formed thereon are washedwith water, dried and then overlapped to each other. The overlappedcopper plates are interposed between steel plates having a hard chromiumplating film formed thereon and then pressed in the following condition;the pressing process can be executed at 180° C. for 30 minutes at apressure of 300 kg/cm². As a result, the two plates are strongly adheredto each other. The silver plating film on the copper plate where it isnot overlapped is exposed to air during the pressing work and then iscolored due to the oxidation of the copper. The joining strength betweenthe two copper plates shows 400 to 500 kg/cm² in accordance with theshearing peeling strength measurement of the tensile strength testmachine.

For the comparison, the following experiment has been carried out: twocopper plates are etched lightly, washed with water, dried and thenoverlapped to each other. The overlapped copper plates are pressed in asimilar condition as above but are not bonded to each other. The copperplate having silver plating film formed thereon is overlapped with thecopper plate washed with water without silver plating. The overlappedtwo copper plates are pressed in a similar condition to that mentionedabove and are not bonded to each other.

The above experiments indicate clearly that the two copper plates arebonded strongly when two copper plates are electrically plated withsilver and pressed to each other at a heated condition.

EXAMPLE 2

A copper foil of 18 μm in thickness is plated at a flat plane withsilver film of 100 angstrom is a similar way to that of Example 1. Thecopper foil in overlapped on the copper plate having a silver platingfilm formed thereon obtained in Example 1 and then pressed in a similarcondition of those of Example 1 except for the pressure of 750 kg/cm².For the easy pressing work, the another side plane of the copper foil isoverlapped with a copper plate of 1 mm in thickness. The copper foil isjoined to the copper plate having silver plating film formed thereon butis not joined to the copper plate by a pressing work.

The pressing work permits the copper foil to join strongly to the copperplate. The peeling test causes the copper foil to be broken and then itis not possible to obtain correctly the joining strength.

EXAMPLE 3

In order to know the effect of the contamination of the surface, thesurface of the copper plate having a silver plating film formed thereonis coated with an epoxy resin (the epoxy resin comprisesepichlorohydrin--bisphonol type of epoxy equivalent of 180) including nocuring agent and pressed in a condition similar to that of Example 1.The result shows a joining strength the same as that of Example 1. It isconcluded that even if the surface of the copper plate having a silverplating film formed thereon is contaminated with resin, there is noeffect on the peeling strength of the resultant bonding.

EXAMPLE 4

A silver plate of 0.5 mm thickness is overlapped with a copper platehaving silver plating film formed thereon and then pressed in acondition the same as that of Example 1. This case shows a strong jointthe same as that of Example 1. It is found at the XMA analysis, as shownin FIG. 5 that the copper atoms diffuse to a surface of the silver platein the depth of a few microns. This result clearly shows the bondingmechanism of the present invention.

EXAMPLE 5

The same combination as that of Example 1 is pressed at 170° C. for onehour instead of 180° C. and 30 min. used in Example 1. The two copperplates join to each other but the strength is very poor. A slightmechanical shaking separates the two copper plates.

EXAMPLE 6

The two copper plates are plated with plating silver film of thickness100 angstroms different from that of the Example 1. Then, the two copperplates are treated in a similar manner to that of Example 1. It ispossible to obtain the joining strength the same as that of Example 1.This indicates that even the thin film of plated silver results ingiving a sufficiently strong joining strength therebetween.

EXAMPLE 7

A phosphor bronze plate of 0.2 mm thick is electrically plated withsilver in a similar way to that of Example 1. The plated silver film hasa thickness of 500 angstrom. The two phosphor bronze plates havingsilver film plated thereon are overlapped to each other and pressed at180° C., for 30 min. at a pressure of 2000 kg/cm². The two phosphorbronze plates are joined to each other. However, the strength is so weakthat the two phosphor bronze plates are easily separated from each otherupon bending at the joint portion. When the pressing is carried out at220° C., it is possible to obtain a strong joint without the loss of theelasticity of phosphor bronze.

EXAMPLE 8

A copper foil on the printed circuit board is plated with gold. Theplated gold film has a thickness of 200 angstrom. After removing thesolder plating layer of the IC lead wire of QFP package by etching thelead wire is plated with copper and further with silver. The platedcopper film has a thickness of 5 μm and the plated silver film has athickness of 500 angstrom. After the IC lead wire is position at a givenpositioned of the printed circuit board, the printed circuit board isheated at 220° C. for one minutes under the lead wire being compressedby a pressing jig. As a result, the lead wire can be strongly joined tothe circuit wiring board.

A heating and cooling cycle test is carried out with the circuitobtained by the present invention and the circuit obtained by theconventional-soldering method. The test result shows that the circuitobtained by the present invention shows a durability longer 2 times thanthat of the circuit obtained by the conventional soldering method.Further, the circuit obtained by the conventional soldering method isbroken upon being heated at 200° C. while the circuit obtained by thepresent invention does not show any change upon being heated at 200° C.

EXAMPLE 9

A copper plate of 2 mm thickness is ground at the surface with a sandpaper of #1200, washed with water and dried. After that, the copperplate is coated with 5% of abietic acid acetone solution and is dried tothe treated copper plate. The two treated copper plates are overlappedto each other and are pressed at 380 kg/cm², for 30 minutes at 180° C.to be joined together. A test piece is prepared from the joined copperplates and is tested with a shear extension apparatus. The joined copperplates show a shear extension strength of 240 to 260 kg/cm² and arejoined to each other strongly.

For a comparison, the copper plates mentioned above are ground with asand paper of #1200, washed with water and dried. The two copper platesare overlapped to each other and are pressed in a similar condition tothat of the above procedure. The result shows no joint between the twocopper plates.

It is confirmed that when the copper plates are coated with 5% ofabietic acid acetone solution and pressed at a heated condition inaccordance with the present invention, the two copper plates can bejoined together strongly.

EXAMPLE 10

The treated copper plate of Example 9 is overlapped with a silver plateof 2 mm thickness. The overlapped copper plate with silver plate ispressed in a condition similar to that of Example 9. Both plates arejoined strongly to each other. The extension test shows that the joinedplate of copper plate and silver plate has a shear extension strength of540 to 600 kg/cm².

The observation by an XMA analyzer indicates that silver and copperdiffuse by several microns to the copper layer and to the silver layer,respectively at the boundary of the joined portion as shown in FIG. 6.The results of an XMA analysis of 240° C., 30 min. pressed copper/silverinterface is also shown in FIG. 7.

EXAMPLE 11

A copper round bar having a diameter of 5 mm is ground at the headingand is coated with abietic acid solution at the flat plane of theheading. Then, after drying, the copper round bar butts at the flatplane against a silver disc having a thickness of 0.5 mm and a diameterof 5 mm. A combination of the copper bar butting against the silver discis pressed by a pressure of 680 kg/cm² for 30 minutes at 195° C. Theresultant copper round bar is tested with a tensile strength test. Thetesting result shows that the copper round bar butting against thesilver disc has a beak down strength of 830 kg/cm². It is verified thatthe copper round bar is strongly joined to the silver disc in accordancewith the present invention.

EXAMPLE 12

After phosphor bronze plate is coated with abietic acid, gold wirehaving a diameter of 1 mm is placed on the phosphor bronze plate. Thecombination of phosphor bronze plate and gold wire is pressed at 195° C.for 15 minutes. As a result, the gold wire joins to the phosphor bronzeplate. The peeling test of the gold wire indicates a peeling strength of4.0 kg/mm². It is found that the gold wire joins strongly to thephosphor bronze plate in accordance with the present invention.

EXAMPLE 13

This Example uses N-laurel aspartic acid β-laurel ester as a copperoxide remover instead of abietic acid used in Example 9. In this case,the remover is scattered on the heated copper plate and is melted. Thetwo copper plates are joined to each other using the melted remover. Itis possible to obtain the joining strength similar to that of Example 9.

EXAMPLE 14

A polymer is prepared by stirring and heating a mixture of acrylic acidmonomer and 0.5% of benzoyl peroxide at 60° C. A 5% aqueous solution ofthe resulting polymer is used as an oxidized layer remover instead ofabietic acid used in Example 9. In this case, it is possible to obtainthe strong diffusion joint in a similar way to that of Example 9.

EXAMPLE 15

The combination the same as that of Example 9 is pressed at 170° C. forone hour instead of 180° C. and 30 min. of Example 1. The two copperplates join to each other but the strength is very poor. A slightmechanical shock separates the two copper plates.

EXAMPLE 16

An electric conductive paste is prepared by dispersing 75 g copperparticles of a spherical form having a diameter 4 to 5 μm (Tanakakikinzoku Co. CRYSTAL COPPER) in a solution of 35 g cellsolve acetateand 25 g rosin. This electric conductive paste is imprinted at a throughhole of a diameter 0.5 mm formed at a given position of a glass-epoxyprepreg. The glass-epoxy prepreg is interposed between two copper foilshaving a thickness of 35 μm and then is pressed into a double layeredprinted circuit board of a thickness of 0.5 mm. The pressing is carriedout at a pressure of 150 kg/cm² at 180° C. for 30 minutes.

The board is etched for the purpose of formation of a given pattern.Then, the through hole is measured with a direct current resistance. Thetesting result indicates that one through hole has an electricresistance less than one m Ω. This electric resistance is kept at acurrent range from a high current to 10 μA.

For comparison, the conventional silver through hole circuit board has athrough hole resistance of 10 m Ω which increases when the electriccurrent is less than 10 mA.

EXAMPLE 17

An electric conductive paste is prepared by dispersing 75 g of spraytype copper particles having a diameter 2 to 15 μm in a solutioncontaining 7.5 g of dimethylor propionic acid and 15 g of an epoxy resin(epoxy equivalent 180) comprising epichlorohydrin bisphenol A type. Thiselectric conductive paste is imprinted in a through hole of a diameter0.5 mm formed at a given position of a glass-epoxy prepreg. Theglass-epoxy plate is interposed between two copper foils having athickness of 35 μm and then is pressed to give a double layered printedwiring board of a thickness of 0.5 mm. The pressing is carried out at apressure of 150 Kg/cm² at 180° C. for 30 minutes.

The board is etched for purpose of formation of a given patter. Then,the through hole is measured with a direct current resistance. Thetesting result indicates that one through hole has an electricresistance less than one m Ω. This electric resistance is kept at acurrent range from a high current to 10 μA.

The circuit substrate is subjected to a heat cycle test from 260° C. to25° C. The test result indicates that the electric resistance does notchange up to 300 cycles test.

For comparison, the conventional silver through hole circuit board has athrough hole resistance of 10 m Ω which increases when the electriccurrent is less than 10 mA. The heat cycle test indicates that theconventional silver through hole circuit board changes in the electricresistance more than 20% through 100 cycles.

This testing result clearly indicates that the double layered printedcircuit board of this embodiment has copper foils at top and bottomplanes joined electrically completely to each other.

EXAMPLE 18

An epoxy prepreg is drilled to make holes of 0.5 mm. The electricconductive paste prepared in Example 17 is applied to an epoxy prepreg.The epoxy prepreg is interposed between the two of the double layeredprinted wiring boards obtained by Example 17 and is pressed at a heatedcondition in order to form a four layer printed wiring board. Theresultant four layer printed wiring board has an electric connectionamong four layers, which is characterized by a stable and a low electricresistance in a similar way to that of Example 17. It is verified that asimple process of hot pressing shown in this embodiment makes itpossible to manufacture a multilayer printed wiring board havingsuperior characteristic in the electrical connection.

EXAMPLE 19

An electric conductive paste is prepared by dispersing 75 g of a spraytype copper powder having a diameter 2 to 15 μm in a solution of 25 gcellsolve acetate and 25 g rosin. This electronic conductive paste isimprinted in a through hole of a diameter 0.5 mm formed at a givenposition of a glass-epoxy prepreg. The glass epoxy plate is interposedbetween two copper foils having a thickness of 35 μm and then is pressedinto a double layered printed circuits board of a thickness of 0.5 mm.The pressing is carried out at a pressure of 150 Kg/cm² at 180° C. for30 minutes.

The board is etched for the purpose of formation of a given pattern.Then, the through hole is measured with a direct current resistance. Thetesting result indicates that one through hole has an electricresistance less than 0.1 m Ω. This electric resistance is kept at acurrent range from a high current to 10 μA.

EXAMPLE 20

An electric conductive paste is prepared by using silver particles ofspherical form having a diameter of 1 μm in replacement of copperparticles used in Example 17. The double layered circuit board isprepared by using the electrical conductive paste mentioned above. A hotpressing in this embodiment is carried out at a pressure of 50 Kg/cm² at190° C. This embodiment also can produce a double layered circuit boardsuperior in the reliability and the electric conductivity. A microscopicobservation at the through hole area of the double layered circuit boardof this embodiment indicates that the silver particles are formed intoone body of silver pillar and that silver and copper execute a mutualdiffusion at the boundary of the copper foil and the, silver pillar.

EXAMPLE 21

In place of a solution of dimethylol propionic acid and an epoxy resin(epoxy equivalent 180) comprising epichlorohydrin-bisphenol A type usedin Example 17, this embodiment uses styrene-maleic acid (1:1) copolymer.In this case, methylethylketone is used for a solvent of the electricconductive paste. This embodiment can produce a double layered circuitboard superior in the electric connection in accordance with the presentinvention.

EXAMPLE 22

In place of a solution of dimethylol propionic acid used in Example 17,this embodiment uses N-stearoil aspartic acid mono-stearyl ester andachieves a double layered circuit board superior in the electricconnection between the circuits on both sides.

EXAMPLE 23

The pressing temperature in Example 17 is changed to 170° C. Thepressing time in Example is changed to one hour. The double layeredcircuit board obtained in such a condition has an electric conductionbetween both sides similar to that of Example 17 but shows a break downat 20 cycles of heat cycle test. These data show that the allowablepressing temperature is 170° C. as a lowest limit.

As described in these embodiments, the method for manufacturing themultilayer printed wiring board according to the present invention issimple in a similar way to that of the silver through hole method, butachieves the multi-layer circuit board superior in the electricalconductivity across both sides similar to that of an electroplatingmethod.

What is claimed is:
 1. A method for preparing a multilayer printed wiring board which comprises:providing at least two foils of Cu or Cu alloy; interposing an insulating adhesive layer between each pair of adjacent foils; making via holes in the each insulating adhesive layer at desired positions; imprinting the via holes with a conductive paste and heatpressing them to electrically connect the foils through the via holes imprinted with the conductive paste, wherein the conductive paste mainly consists of conductive metal particles selected from the group comprising Ag, Cu and alloys thereof and a metal oxide remover; wherein the heat pressing is carried out at a temperature higher than about 170° C. whereat the Cu atoms can be diffused.
 2. The method for preparing a multilayer printed wiring board according to claim 1, wherein the metal oxide remover is selected from the group comprising carboxylic acids and polymers of oligomers having carboxyl groups.
 3. The method for preparing a multilayer printed wiring board according to claim 2, wherein the metal oxide remover acts as the binding resin.
 4. The method for preparing a multilayer printed wiring board according to claim 1, wherein the conductive metal particles are of spherical form.
 5. The method for preparing a multilayer printed wiring board according to claim 1, wherein the step of imprinting the via holes with a conductive paste comprises a step of drilling the insulating adhesive layer sandwiched between a pair of peelable films; a step of imprinting the conductive paste on the peelable film; and a step of peeling the peelable films. 